1. Field of the Invention
The present invention relates to a balanced-type mixer for use in a transmitter-receiver at millimeter-wave and microwave frequencies.
2. Description of the Related Art
A diode mount structure in a nonradiative dielectric line (hereinafter referred to as an "NRD guide") and a mixer using such structure are disclosed in "Nonradiative Dielectric Line Circuit Elements using Beam Lead Diodes", Journal of The Institute of Electronics, Information and Communication Engineers, by Kuroki and Yoneyama, C-I, Vol. J73-C-I, No.2, pp.71-76, February 1989.
This mixer is structured in such a way that a coupler is formed based on an NRD guide, and a circuit board having diodes mounted thereon is perpendicularly interposed between dielectric strips in order to couple the diodes to the NRD guide.
However, in the structure shown in the above reference, since a circuit board having diodes mounted thereon is disposed perpendicular to the direction along the length of a dielectric strip, it is difficult to fix the circuit board within the apparatus and the circuit board is likely to incline; therefore, ease of mounting is poor. Further, since a thin plate having a high dielectric constant must be inserted into an NRD guide or a gap must be provided to achieve matching, design and manufacturing are not easy.
Further, the coupler of the NRD guide has problems in that, as shown in FIG. 12, the frequency at which the power distribution ratio becomes equal is at one point. The farther away from that frequency, the more the balance of the power distribution ratio is distorted, and the advantages of the balanced-type mixer, such as suppression of image signals and cancellation of the effect of the drift of local signals, are decreased. In addition, in a conventional balanced-type mixer, since a coupler based on an NRD guide is formed, the line length of the NRD guide is increased, and losses in the coupler portion become a problem.